public class mytest {

    public static void main(String[] args) {
        int drink = 0;
        int empty = 15;
        while(true){
            int full;
            full = empty / 3;
            drink += full;
            empty = empty % 3+full;
            if(empty == 1 || empty ==0){
                break;
            }else if(empty == 2){
                drink++;
                break;
            }
        }
        System.out.println(drink);
    }
}
